Compressed air pressure for valves 2

[ChemGuy2016]

I am wondering, what is a common range of air pressure required to actuate control valves? Butterfly and globe.

Thanks!

 

[mk3223]

Depend on many factors, such as the process pressure the valves used for the control, the size, function, and type of the valves, etc, the air supply pressure for the valve actuators could be from 40 to 80 psig.

 

[don1980]

There are some important reasons to use a higher pressure for instrument air systems:
[ul]
[li]Higher air pressure allows the use of smaller, less expensive valve actuators.[/li]
[li]Higher pressure provides more motive force for faster response time from the control valves.[/li]
[li]Higher pressure minimizes the pressure fluctuations that are experienced in the instrument air system, thus preventing occasional sluggish response from some of the valves.[/li]
[/ul]

I suggest operating your instrument air at about 100 psig, but consult with an instrument engineer on this.

 

[shvet]

API RP 552-1994
"21.2 Air supply system
For proper unit instrumentation operation, instrument air shall be ... at minimum pressure of approximately 100 psig (700 kPag) at the inlet to instruments air header.
...
21.2.11 Distribution system
Lines in the distribution system should be sized a manner that the maximum pressure drop between the dryer outlet and the most remote consumer does not exceed 5 psi (35 kPa) when all consumers are taking air at maximum rates."

All it means 665 kPag at the inlet to control valve's actuator.

 

[ChemGuy2016]

don1980: Why does higher air pressure allow for smaller valve actuators? I would think that smaller actuators would require a lower air pressure.

 

[bimr]

Assume you are using a cylinder actuator. The energy of the air is applied to the end of the cylinder. The operating friction internal to the cylinder is negligible and may be ignored. The total force applied is the air psig * area of the cylinder end. Bigger cylinder equals more force, smaller cylinder equals less force.

 

[don1980]

Chemguy - The actuator provides the power for manipulating the position of the control valve. The air generates this force by acting upon a piston or a diaphragm within the actuator. To generate a given torque value, the required size (required surface area for the air to act on) depends on the pressure of the air. A given torque value can be generated by a big actuator using low-pressure air, or it can be generated by a small actuator using high pressure air.

 

[ChemGuy2016]

don1980 - I understand now. The actuating force required by the valve remains constant, so a larger piston requires lower air pressure. I see.

So looks like we usually deal with up to 100 psi, and that would be for a small actuator. 40-60 psi for a larger one.

Thanks everyone!

 

[Compositepro]

In the early '80's I worked in a chemical plant that was built in the '50's, which used pneumatic controls. The control valves operated on 3-15 psi air and all control signals were 3-15 psi so that the output of a controller directly operated the valve. The controllers were pneumatic analog computers, and many mathematical functions were possible. The signals were transmitted through 1/4" copper tubing over hundreds of feet. Instrument air was 100 psi and at each valve or controller there there was a regulator that reduced pressure to 20 psi. This system of control was very robust and reliable, and very safe around flammable chemicals.

This relatively low air pressure did require the use of large (~12" diameter) rolling-diaphragm valve actuators.